I need a solution of some software or built-in feature that allows me to stick a Window somewhere. I mean, it's any program that is not in fullscreen, and I position it in the top-left of my screen, for example, I want that when I click inside of a Window, my browser for example, it remains there.

A solid example:
I want to let some youtube video in a small separate Window of my browser, and want to be on Facebook while I watch the video, but splitting the screen loses lot of space in my browser, so I'd prefer to let the video in the top-left screen, with the window resized, and still be able to browse without letting the video go behind my fullscreen when I click it